1. Field of the Invention
The present invention relates to capacitor charging apparatuses, electronic flash (stroboscope) apparatuses, and cameras having an internal electronic flash (stroboscope) apparatus.
2. Description of the Related Art
Forward-type converters mainly have been used for booster circuits in conventional electronic flashlight (stroboscope) apparatuses. Forward-type booster circuits are simple in circuit structure and are affected little by variations of oscillating transformers. Therefore, they have been widely used.
As cameras have been made more compact, low-capacity batteries mainly have been used. In contrast, high guide numbers are required, which means a large amount of flash light is required. Therefore, flyback-type converters, which are more efficient than forward-type converters, has been started. In general flyback-type converters (hereinafter just called flyback converters), oscillation is controlled such that the primary current of an oscillating transformer is detected and the primary current of the oscillating transformer is interrupted at a predetermined current (hereinafter called Ip). When a main capacitor is charged by a flyback converter, efficient charging is achieved by a low current. Low-current charging, however, requires a long charging time, generates a release time lag at time of photograph capture, and therefore, fails to provide a good chance of timely pressing the shutter release. To avoid such a case, and in addition, to reduce the charging time, a relatively high current flow needs to be generated.
Since the amount of current flow differs between a case when a new battery is used and a case when a partially-used battery is used, when the limit current of the primary current (hereinafter called a primary limit current) of the converter is set to Ip1 with a new battery as shown in FIG. 9, it takes a long time to reach Ip1 with a partially-used battery and the amount of current indicated by hatching between t1 and t2 is wasted. In FIG. 9, L1 indicates the primary current obtained with a new battery, and L2 indicates the primary current obtained with a partially consumed battery.
When the primary limit current is set to Ip2 with a partially used battery, Ip2 cannot be a large value and a long charging time is required.
In a circuit in which the limit current Ip is detected at a constant current irrespective of the state of a battery, if the battery is consumed, the efficiency of the flyback converter is reduced, and finally, the battery current cannot be controlled.
This point will be described below by referring to FIG. 15. As a battery is consumed, the output voltage of the battery is reduced and the internal resistance of the battery increases. As shown in a waveform xe2x80x9cbxe2x80x9d or a waveform xe2x80x9ccxe2x80x9d in FIG. 15, the primary current of the oscillating transformer has a drooped waveform (approaching horizontal asymptote). This is because the equivalent circuit of the flyback converter is formed of a series circuit of a power supply having the voltage of the battery, the internal resistor of the battery, a loop resistor, and an inductor. A current I flowing through the circuit is expressed by the following equation.
I={E/(Rbat+R)}*[1xe2x88x92exp{xe2x88x92(Rbat+R)*t/L})]
where, E indicates the voltage of the battery, Rbat indicates the internal resistance of the battery, R indicates the loop resistance, and L indicates the inductance of the primary winding of the oscillating transformer.
As the battery is consumed, the primary current of the oscillating transformer becomes a drooped waveform as shown by the waveform xe2x80x9cbxe2x80x9d in FIG. 15. A hatched portion in FIG. 15 is almost a loss and efficiency decreases. As the battery is further consumed, the primary current of the oscillating transformer continues to flow (because it does not reach the limit current Ip), and a charging operation is not performed, as shown by the waveform xe2x80x9ccxe2x80x9d in FIG. 15.
To solve this problem, a method can be considered in which the limit current Ip is appropriately controlled according to the state of the battery. In this case, however, it is necessary to change a threshold voltage of the converter, which controls the limit current Ip. Therefore, a D/A converter is required in a control circuit, and thereby the circuit structure of a camera becomes complicated.
The present invention has been made in consideration of the foregoing conditions. Accordingly, it is an object of the present invention to provide a capacitor charging apparatus which has a simple circuit structure and efficiently charges a capacitor irrespective of the state of a partially consumed battery.
Another object of the present invention is to provide an electronic flash (stroboscope) apparatus which has a simple circuit structure and efficiently charges a capacitor irrespective of the state of a partially consumed battery.
In one aspect, an object of the present invention is achieved by providing a capacitor charging apparatus including a DC/DC converter for stepping up the voltage of a battery; a capacitor charged by the DC/DC converter; a detection circuit for determining whether the primary current of the DC/DC converter reaches a predetermined limit current; a control circuit for controlling the DC/DC converter according to a detection signal from the detection circuit; and a battery-information detection circuit for detecting battery information, wherein the control circuit performs a predetermined calculation using battery information from the battery-information detection circuit to determine the predetermined limit current so as to vary the primary current of the DC/DC converter and control the DC/DC converter every time the capacitor is charged by the DC/DC converter.
The capacitor charging apparatus may be configured such that it further comprises a time measuring circuit for measuring the maximum time for which the primary current can be caused to flow, and when the time measuring circuit has counted up to the maximum time before the primary current reaches the predetermined limit current determined by the predetermined calculation, the control circuit controls the DC/DC converter using a count-up signal.
In the capacitor charging apparatus, the count-up signal of the time measuring circuit may correspond to a calculated value not exceeding the saturation current of an oscillating transformer in the DC/DC converter, related to the internal resistance of the battery, or to a determined fixed value not exceeding the saturation current of the oscillating transformer.
In the capacitor charging apparatus, the DC/DC converter may be a flyback converter.
In the capacitor charging apparatus, the battery information may be the no-load voltage of the battery, or a voltage obtained at a predetermined load.
In the capacitor charging apparatus, the predetermined limit current may be calculated according to the internal resistance of the battery, obtained from the battery information.
In the capacitor charging apparatus, the control circuit may comprise a microcomputer which includes an A/D converter and a D/A converter that outputs the predetermined limit current.
In another aspect, an object of the present invention is achieved by providing an electronic flash (stroboscope) apparatus including the above capacitor charging apparatus and a discharge tube which emits light by discharging energy accumulated in the capacitor.
In still another aspect, an object of the present invention is achieved by providing a capacitor charging apparatus including a DC/DC converter for stepping up the voltage of a battery; a capacitor charged by the DC/DC converter; a detection circuit for determining whether the primary current of the DC/DC converter reaches a predetermined value; a control circuit for controlling the DC/DC converter; and a battery check circuit for detecting the state of the battery voltage before a charging operation of the capacitor is started, wherein the control circuit switches control of the DC/DC converter according to the state of the battery voltage detected by the battery check circuit, between a control operation performed according to a detection signal from the detection circuit and a control operation performed according to a signal having a fixed pulse width.
In the capacitor charging apparatus, the control circuit may switch control of the DC/DC converter from a control operation performed according to a detection signal from the detection circuit to a control operation performed according to a signal having a fixed pulse width when the internal resistance of the battery increases or when the battery voltage is reduced to a predetermined voltage or less.
In the capacitor charging apparatus, the primary current of the DC/DC converter, obtained when the DC/DC converter is controlled according to a signal having a fixed pulse width may be lower than that of the DC/DC converter, obtained when the DC/DC converter is controlled according to the detection signal from the detection circuit.
In yet another aspect, an object of the present invention is achieved by providing an electronic flash (stroboscope) apparatus including the above capacitor charging apparatus and a discharge tube which emits light by discharging energy accumulated in the capacitor.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.